The present invention relates to a hydrophobic polyurethane elastomer prepared by combining a polybutadiene diol and an aliphatic or cycloaliphatic diisocyanate to form an isocyanate terminated prepolymer and chain extending the prepolymer with a diol.
It is well known to prepare isocyanate terminated polyurethane prepolymers from polyether and/or polyester polyols and aromatic diisocyanates. Furthermore, it is also well known to prepare polyurethane elastomers by chain extending these prepolymers with low molecular weight diols. The resulting polyurethanes have excellent mechanical properties, but are rather hydrophilic, which can limit their utility in certain moisture sensitive applications.
Hydroxyl terminated polyols with very non-polar backbones (e.g., hydroxyl functional polybutadiene) can be used to introduce hydrophobicity into polyurethane elastomers. This results in improved resistance to hydrolysis over polyether or polyester polyol based polyurethanes. However, the mechanical properties of the resulting polyurethane elastomers are generally rather poor when they are prepared using the conventional diisocyanates and diol chain extenders. In particular, they suffer from poor tear strengths and low elongation at break. This combination of properties limits their utility in applications requiring good resistance to tearing or stretching.
The modest mechanical properties of the polybutadiene containing polyurethane elastomers are probably at least partially due to the high functionality of the commercially available polybutadiene polyols (2.2 to 2.8 functional). In contrast, the polyether or polyester polyols used to make polyurethane elastomers with excellent mechanical properties generally have functionalities very near 2.0.
Hydroxy-terminated polybutadienes having a functionality of 2.0 are disclosed in Chung, et al. (U.S. Pat. No. 5,247,023), Grubbs, et al. (U.S. Pat. No. 5,750,815) and Nubel, et al. (U.S. Pat. Nos. 5,512,635, 5,559,190, 5,519,101 and 5,403,904). These diols, when formulated into polyurethane systems with aliphatic or cycloaliphatic diisocyanates, may offer the opportunity to produce hydrophobic polyurethane elastomers with improved mechanical properties over the current state of the art.
The present invention relates to a hydrophobic polyurethane elastomer comprising:
A) An isocyanate terminated prepolymer having an isocyanate content ranging from 4 to 12 wt. % NCO comprising the reaction product of:
i) an OH terminated homopolymer of butadiene having a molecular weight ranging from 1000 to 4000 and an OH functionality of from 1.9 to 2.1;
ii) an aliphatic or cycloaliphatic diisocyanate.
B) A diol chain extender having a molecular weight ranging from 62 to 400.
In earlier experiments, it was shown that substitution of the dihydroxyl terminated polybutadiene of the type described above for a commercial polybutadiene polyol (Elf Atochem""s R-45 HT) in aromatic diisocyanate based prepolymers, followed by chain extension with conventional diol chain extenders like 1,4-butanediol, failed to produce a useable hydrophobic elastomer. Surprisingly, it was determined that with the hydroxy terminated polybutadiene described above, an aliphatic or cycloaliphatic diisocyanate and a diol chain extender, one could produce useable elastomeric materials. In addition, these hydrophobic elastomers were optically clear, light stable and showed dramatic improvement in tear strength and elongation at break over conventional polybutadiene polyol based elastomers.